The present invention relates to a cold sealable adhesive and more particularly relates to a cold sealable adhesive containing synthetic latexes.
Cold sealable adhesives form a class of adhesives which work by forming an adhesive-adhesive bond without the need for the application of heat. In the art, this is often referred to as an A-A seal. Thus in order to bond two substrates, a layer of the cold sealable adhesive is applied to both the substrates, then the substrates are brought together under pressure in a manner such that the layers of adhesive are nipped together. Cold sealable adhesives differ from pressure sensitive adhesives, where two substrates are bonded by coating only one of the substrates with adhesive, then bringing the substrates together. In the art, this is often referred to as an A-B seal.
Plastic, foil and paper packaging structures are used in the food packaging, medical and industrial packaging industries for the wrapping and protection of the vast majority of foodstuffs, medical articles, and for the protection of general goods such as books and cassettes These structures may be mono webs or laminates of different plastics, plastics and metal foils, or plastics and paper. Materials are wrapped in the structures and these are subsequently sealed either by welding with heat and pressure, or via a heat seal adhesive with heat and pressure or by the application of pressure alone. Non woven materials are used in the manufacture of bandages for example for field dressings where the bandage is wrapped around the wound and then sealed or fastened by the application of pressure alone.
The wrapping of confectionery, biscuits, cakes and ice cream is a special case in the field of food packaging in that preferably the sealing process is carried out at ambient temperature. This allows for significant increases in line speeds thus providing for more economical production. In addition it serves to prevent damage to products (particularly chocolate based products) during the wrapping process, especially in the event of a line breakdown when materials may be left standing for some time.
Traditionally such cold sealing adhesives are water based and derived from natural rubber latex coupled with a synthetic polymer which is usually acrylate based. The latex is primarily a dispersion of cis-1,4-polyisoprene which is stabilised with a natural water soluble protein fraction.
Cis-1,4-polyisoprene has the following structure: 
Natural rubber latex exhibits several unique properties that make it suitable for use in such cold sealing adhesives. More specifically, it readily seals to itself but does not produce a tacky film and hence will not block when reeled up to the opposite surface.
To date, synthetic cold sealing adhesives have usually been based on pressure sensitive technology utilising acrylic polymers. The main drawback with this approach has been the tacky nature of the adhesive films produced and subsequent blocking in the reels and noise produced when the converted film is unwound at the packing station
This present invention relates to the use of synthetic latexes for the production of an adhesive which may be applied to plastic, paper, foil, coated films, or non woven substrates, and subsequently dried producing a tack free film. At a later date the adhesive film may be used to produce a bond by sealing the adhesive film to itself by pressure alone.
The present invention provides an entirely synthetic solution to the problems in the prior art. Specifically, it is an aim of the present invention to provide a cold sealable adhesive which contains mostly synthetic products, and which preferably consists entirely of synthetic products, but which does not have the tackiness problems associated with the synthetic adhesives based on acrylic polymers. Thus, the adhesive provided by the present invention will seal to itself with the application of pressure in the absence of heat The adhesive will also provide a tack free film which will not block when wound up or produce unacceptable noise whilst being unwound from a release lacquer or homopolymer untreated film. In addition, the adhesive will wet out and adhere to a number of substrates including plastics, coated plastics, paper type films, and non wovens, and is capable of being applied on a high speed printing press.
We have found that the use of synthetic cis-1,4-polyisoprene as a cold seal adhesive solves many, if not all, of the problems in the prior art. In addition, we have found that the use of synthetic cis-1,4-polyisoprene as a cold seal adhesive provides an unexpectedly improved seal strength over the natural product, and provides better printability than the natural product.
According to one aspect of the invention we provide a cold sealable adhesive which comprises synthetic cis-1,4-polyisoprene, and which does not contain any natural cis-1,4-polyisoprene. Thus, the adhesive comprises man-made cis-1,4-polyisoprene, and does not contain any such material derived from a naturally occurring source, such as a rubber tree.
The synthetic cis-1,4-polyisoprene used in the present invention is obtained by a synthetic manufacturing process, i.e., it is not obtained by treatment of or isolation from natural rubber latex.
According to another aspect of the invention there is provided the use of synthetic cis-1,4-polyisoprene as a cold sealable adhesive. Advantageously, the synthetic cis-1,4-polyisoprene is provided in the form of a dispersion in water.
According to another aspect of the invention there is provided a method of sealing two substrates together comprising the steps of: coating each substrate with a layer of a cold sealable adhesive comprising synthetic cis-1,4-polyisoprene; and bringing the adhesive layers of each substrate into contact to adhere the substrates together. More preferably the cold sealable adhesive comprises a cold sealable adhesive composition as described above. In general, pressure would be applied to assist the bonding of the substrates.
According to another aspect of the invention there is provided a method of sealing two parts of a substrate together, comprising the steps of: coating both parts of the substrate with a respective layer of a cold sealable adhesive comprising synthetic cis-1,4-polyisoprene; and bringing the adhesive layers of each part of the substrate into contact to adhere the parts of the substrate together. More preferably the cold sealable adhesive comprises a cold sealable adhesive composition as described above. In general, pressure would be applied to assist the bonding of the parts of the substrate.